The proposed research concerns the isolation of purification of the human group I pepsinogens and group II pepsinogens and their component fractions, their determination by radioimmunoassay in serum and urine during basal and stimulated conditions in patients with and without peptic ulcer, the investigation of the effect of ionic composition and ionic strength on the proteolytic activities of the group I pepsins and the group II pepsins, and the optimal conditions for their proteolytic assay. To be studied are the factors and mechanisms involved in the betazole-induced decrease in serum group I pepsinogen levels after vagotomy and the clinical application of this response to the evaluation of risk for recurrent ulcer in the postoperative patient. In addition, the factors involved in the heterogeneity of renal clearance of the group I pepsinogens and the relationship of gastric group I pepsin secretion to serum levels and urinary output of group I pepsinogen levels will be studied. The distribution of serum group II pepsinogen levels in patients with and without ulcer will be defined.